Manual gear boxes, contrary to automatic transmissions, are preferably designed as synchronized layshaft transmissions which have, in the direct speed, connected the drive shaft directly with the aligned driven shaft, while in the other speeds the flux of force is guided from the drive shaft to the driven shaft via a layshaft. The pairs of gears of the different speeds are constantly meshing and for this reason the gears arranged on the driven shaft are seated to rotate about the latter. For the purpose of putting in a speed, the corresponding gear and the driven shaft are positively coupled in the conventional manner. To facilitate the shifting process for the driver, the positive clutches are provided with preliminary friction clutches which serve as synchronizing mechanisms and ensure the synchronized motion of the shaft and the gear before they are positively coupled. Transmissions of this type have replaced in passenger cars the unsynchronized transmissions heretofore used. Initially, only the higher speeds were provided with synchronizing means, but later the first speed was also synchronized as it was found that it does not so seldom occur that the first speed is engaged while the car is moving. The reverse motion is usually not provided with synchronizing means, which means that its shifting clutch lacks the preliminary friction clutch. The reason for this is to be seen in the fact that due to the reversal of the travelling direction the car is always stopped before the reverse motion is put in. When shifting into the reverse motion, the drive shaft is indeed stationary--provided the motor car is not moving--but the gear for the reverse motion which is rotatably seated on the drive shaft is still moving as it is meshing with the gear seated on the layshaft (via an intermediate gear for the reversal of the sense of rotation) so that it revolves at a speed proportional to that of the drive motor. Although the driving connection between the drive motor and the layshaft is interrupted when the main clutch has been actuated, the individual parts continue to rotate as a result of the mass moment of inertia and come to a standstill only gradually as a result of the frictional losses, which in the case of heated-up transmissions may take many seconds as in this condition the oil is very thin.
Now, when the shifting clutch in the transmission is operated shortly after the driver has actuated the main clutch, the flanks of the teeth of the shifting clutch come abruptly to bear against each other so that the still revolving parts are suddenly stopped. Although this does not actually impair the service life of the transmission, the resulting noise is rather disturbing.
Of course, this drawback could be eliminated in the same manner which is also employed for the other gears, namely, by synchronizing means in the form of a preliminary friction clutch. But this would entail the risk that the reverse motion could be put in erroneously and unintentionally, and contrary to the consequences of such an error in the case of the other gears, the subsequent engagement of the main clutch would lead to damages or destructions in the driving system of the car if the latter were moving at a not inconsiderable forward speed.
It is already known to provide all gears of agricultural tractors with synchronizing means. But these tractors when being driven on the soil do not move for a longer period when the clutch is shifted. Therefore, no risk exists that at the subsequent engagement of the main clutch the tractor is moving at considerable speed. Furthermore, the main clutch is not arranged between engine and transmission, but is arranged between transmission and the driven axle. (Lastauto-Omnibus, May 1967.)
Finally, industrial vehicles are known for the transportation of heavy loads within buildings which are provided with transmissions with several forward and reverse gears, all of which are synchronized. In order to avoid damages in the driving system by rapid changing from forward to reverse gears, the transmission is provided with a mechanically or electrically actuated stop, the actuation of which depends on the vehicle speed. The stop is arranged to block up the operation of the manually operable gear lever (German Auslegeschrift 11 77 948).
Now, it is the object of the present invention to provide a variable speed transmission of the type described above in which the reverse motion can be put in without disturbing noise, while on the other hand any damages are safely excluded in case the reverse motion should be put in unintentionally while the car is moving forward.
Another object of the invention is to provide a variable speed transmission with a safety stop which automatically makes it impossible for the driver to shift into the reverse gear when the car is moving forward with substantial speed. Another object of the invention is to provide a stop of a purely mechanical type which is operated by centrifugal force. It is a further object to provide a transmission with a stop acting immediately upon a shifting clutch and, therefore, sources of errors, such as those that may arise for instance from distortions of the car body, are safely avoided.
Another object of the invention is to provide a stop actuated by centrifugal force provided by revolving parts which are connected with the driving axle of the car.